(1) Field of the Invention
The present invention relates to a method for improving the cold flow of hydrocarbon fuel oils.
(2) Related Art Statement
The oil prices have largely increased since the oil shock, which has greatly influenced all the industrial fields. Owing to this, many industries such as the steam power generation industry, the iron and steel industry, and the cement industry have tried to reduce or remove dependency of oils. As a result, demands for heavy oils mainly consumed in these industries have greatly been reduced. On the other hand, since middle or light fuel oils are mainly consumed in the living life and the transportation field, there is a tendency that demands therefor have increased to the contrary.
To cope with such changes in oil supply and demand situations, a number of countermeasures have been considered and practically carried out. As one of the countermeasures, a part of the heavy distillate is tried to be used for middle fuel oils. In particular, it is a strong tendency that the middle distillate fuel oils such as diesel gas oils and heating gas oils have become heavy.
As compared with the conventional fuel oils, such heavy middle distillate fuel oils contain a greater amount of paraffins having greater molecular weights, so that they are likely to precipitate the paraffins at low temperatures, and lose their cold flow at relatively high temperatures. Since large crystal grains of the paraffins are formed even at a temperature range in which the cold flow is maintained, filters or pipe lines in fuel oil systems of diesel engines or the like are plugged to interrupt smooth supply of the fuel oil.
In order to solve the above-mentioned problems, various cold flow improvers have heretofore been disclosed. For example, there are recited condensation products between chlorinated paraffins and naphthalene (U.S. Pat. No. 1,815,022), polyacrylates (U.S. Pat. No. 2,604,453), polyethylene (U.S. Pat. No. 3,474,157), a copolymer between ethylene and propylene (French Pat. No. 1,438,656), and a copolymer between ethylene and vinyl acetate (U.S. Pat. No. 2,048,479).
In the pour point test (JIS K 2269), these cold flow improvers exhibit relatively excellent pour point-lowering action. However, in the cold filter plugging point test (IP 309) for judging plugging of fuel oil filters at low temperatures, almost no effect is obtained in many of them. Particularly, the number of cold flow improvers which are effective for fuel oils containing much paraffins having high molecular weight is few.
It is difficult for the pour point test method to anticipate plugging of fuel oil filters due to paraffin crystal grains, which occur at temperatures much higher than the pour point. For this reason, the cold filter plugging point (hereinafter abbreviated as "CFPP") test has been contrived as an improved method of the conventional pour point test. It is an actual situation that the CFPP test are widely employed as a simple test method for evaluating practical low temperature cold flow of fuel oils.
The present inventors had repeatedly made studies to solve the problems regarding the abovementioned low temperature cold flow of the fuel oils. As a result, they found out that the CFPP is very effectively lowered by ester compounds in which an aminoic nitrogen atom is located in the center and in which a straight chain saturated hydrocarbon group is bonded to a site relatively near the aminoic nitrogen atom via an ester bond. This led to inventions disclosed in U.S. Pat. No. 4,509,954, European Pat. No. 117,108, Canadian Pat. No. 1,218,233, etc.
Although their inventions offer excellent cold flow improvers which effectively lower the CFPP of the above-mentioned fuels by a small addition amount, the kinds of fuels upon which the most excellent effects are afforded by the ester compounds of this type is limited, and optimum ester compounds need to be selected depending upon the kinds of fuel oils. For instance, it was found that an ester compound which exhibited the most excellent effect for No. 3 gas oil (guaranteed temperature: -20.degree. C.) specified in JIS K 2204 could not be said to be the most preferable for No. 1 gas oil (guaranteed temperature: -5.degree. C.), specified in JIS K 2204, and that another ester compound was the most preferable for the latter.